1. Field of the Invention
The present invention relates to a signal conditioning apparatus that serves to eliminate electrical and/or magnetic interferences which develop during transmission of electrical signals. In addition, the signal conditioning apparatus also serves to drive output conductors in such a way as to overcome the adverse effects of loading caused by the output conductors on the signal source.
2. Description of the Related Art
Conductors that provide an electrical connection between devices in a system are often the source of many types of electrical interference. Magnetic fields, electric fields and electro-magnetic or radio frequency fields are known to interfere with the fidelity of signals conveyed over conductors which are subjected to those fields. Furthermore, the ground or reference conductor of a typical signal carrying pair of conductors is often connected to different local ground potentials between one end of the conductor as compared to the other, and currents are known to flow in such conductors which then produce voltage drops on that conductor which also interfere with the fidelity of the signals being conveyed. In addition, these conductors; especially when very long, present loads to the signal source that may adversely effect the fidelity of the signal.
The problems of conveying signals over conductor pairs are well known. The conveyance of signals, especially between powered devices, is often plagued by electrical and/or magnetic interference. One method employed to reduce these interferences modulates the signal so that it can be easily separated from the interference, and then demodulates at the destination. For example, an analog to digital converter can be utilized to convey digital impulses over the connecting conductors instead of analog voltage potentials. The destination device in such instances must then convert the signal back to an analog signal potential. Such approaches, while very effective, can be very costly, and require extensive circuitry at both the sending and receiving ends of the conductors. Such methods are exemplified by U.S. Pat. No. 4,922,536.
One common method to reduce these interferences is to convey such signals in a differential manner. A common approach utilizes a three conductor shielded cable where two of the conductors deliver the signal and its arithmetic inverse, and the third conductor, usually a shield, conveys the ground reference potential voltage. The conditioning circuit, usually placed at the destination end of the conductors, forms the difference between the potential of the first signal carrying conductor and the second signal carrying conductor. In theory, both conductors are subject to the same interferences, and the subtraction of the signals as conveyed will eliminate the common mode noises. This approach, while very effective in eliminating most interference is nevertheless expensive and difficult to implement. To adapt this approach in the general case of processing signals between subsystems requires active circuitry at the sending end to form the inverse signal, and active circuitry at the receiving end to subtract the signals. Multiple conductors are also required to be contained within a single shield, which is more costly than conductors having only one conductor surrounded by a shield.
One source of interference in the conveyance of these differential signals between electronic subsystems is referred to as the ground loop. Because it is common for there to be multiple electronic paths between the reference potentials of each subsystem, and since such paths commonly include sources of interference, these alternative paths are often responsible for the interference present in those systems. Such ground loops are generally overcome by eliminating any electrical connection by conductors between the subsystems. Ralph Morrison in his book entitled "GROUNDING AND SHIELDING TECHNIQUES IN INSTRUMENTATION", Ralph Morrison, 3rd Ed., 1986, Wiley-Interscience, pgs. 69-71, teaches art that eliminates the effects of the electrical connections between subsystems that convey their signals by differential means through the use of tandem differential amplifiers powered by electrically isolated power supplies. A first differential amplifier calculates the difference between the signals being conveyed, and the second differential amplifier adds the reference potential of the destination to the result of the first differential amplifier. The result is that the reference potentials of the source of the differential signal may differ from the reference potential of the destination without effecting the expression of the signal at the destination. However, such an approach is not easily adapted to electronic systems consisting of single ended two wire signal conductors. Consequently, this approach suffers from the same limitations as devices that convey signals by differential means.
Therefore, a need exists for a signal conditioning system which receives electrical signals, and suppresses interference, such as electric and/or magnetic fields which may interfere with the re-transmission of the received electrical signals.